1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device that uses a chiral nematic liquid crystal having two metastable states. More specifically, it relates to a drive method that improves the speed of writing. The present invention also relates to a drive method that can compensate the drive voltage to cope with irregularities in the threshold value of the liquid crystal that are specific to each liquid crystal panel, and provide temperature compensation of the drive voltage. The present invention further relates to a drive method that ameliorates the imbalance between the voltages of two different drive waveforms and makes it possible to turn the drive circuitry into an Integrated Circuit chip (IC).
2. Related Art
The driving of a liquid crystal that has bistability, using a chiral nematic liquid crystal medium, has already been disclosed in Japanese Laid-Open Publication 1-51818, which includes descriptions of initial orientation conditions, two metastable states, and a method of switching between these two metastable states.
However, the drive method described in Japanese Laid-Open Publication 1-51818 has many implicit problems when it comes to practical implementation. For example, the above mentioned publication discloses two methods for switching between metastable states.
The first method obtains the two metastable states as follows: a 360.degree. twist orientation state is obtained by using a toggle switch to suddenly turn off the voltage (60 Hz, 15 V peak-to-peak) applied to the liquid crystal; and a 0.degree. uniform orientation state is obtained by using a variable-voltage device to allow the voltage applied to the liquid crystal to slowly fall over approximately 1 second.
The second method is as follows: if a high frequency of 1500 kHz is applied directly to the liquid crystal after a low-frequency field is turned off, a 360.degree. twist orientation state is enabled. If the 1500-kHz high-frequency field is applied after a delay of approximately 1/4 second after the same low-frequency field has been turned off, a 0.degree. uniform orientation state is achieved.
The first method is completely impracticable; it can never progress beyond simple verification in the laboratory. When the present inventors came to experiment with the latter method, they discovered that, if a high-frequency field is applied after a delay of approximately 1/4 second after the low-frequency field has been turned off, the same 360.degree. twist orientation state is achieved, and it was thus impossible to switch between the two metastable states.
Further, Japanese Laid-Open Publication 1-51818 states nothing about a matrix display which is currently most appropriate for practical use and has high quality as a display device, and disclosed nothing about a drive method for such a device.
In U.S. patent application Ser. Nos. 08/059,226 and 08/093,290, the inventors of the present invention disclosed a method of controlling the backflow generated in liquid crystal cells to ameliorate the above described fault. However, these disclosures did not have as their objective, shortening the time required to write each line. Therefore, the time required for each embodiment of the above-mentioned disclosures to write one line of a matrix display is 400 .mu.s, so that writing of 400 or more lines would necessitate a total of at least 160 ms (6.25 Hz). This is impracticable because it would result in flickering of the display.
In general, irregularities in the drive characteristics that are induced during the process of fabricating a liquid crystal display panel include differences in drive characteristics that depend on position within any one display panel and differences in drive characteristics between different display panels caused by differences between manufacturing lots. Therefore, to ensure that a whole liquid crystal display screen can be used with its display quality always optimized, it is necessary to provide subtle control of the drive voltage to match each panel. In addition, even if the optimal adjustment has been achieved by some method, new changes in the drive conditions are likely to be caused by variations in the surrounding temperature, so it is essential to provide further adjustment to match temperature variations.
Differences in the threshold value of the drive voltage within a single panel are shown in FIG. 49. Since the drive voltage varies in this manner in response to small differences in orientation state or variations in cell gap, an optimal adjustment of the drive voltage must be done for each panel to accommodate its worst area. Variations in drive voltage with respect to temperature variations that are assumed likely to occur for a matrix drive are shown in FIG. 18. The gradient of the curve with respect to temperature is low at 0.02 V/C, but, if a voltage change ratio of 0.56%/.degree.C. is considered based on a drive voltage of 25.degree. C., this becomes fairly large at 19.6% over an actual temperature range of 5 to 40.degree. C. This looks promising for compensation to the optimal display in practical use.
When it comes to driving a liquid crystal having a memory capability, a reset pulse of a comparatively large absolute value must be applied to the liquid crystal in order to cause a Freedericksz transition in the liquid crystal molecules. This causes a large imbalance in the voltage ratio between the scan and data signals during matrix drive. This imbalance is expected to lead to large problems in the configuration of specific drive circuits, or the turning of such circuits into ICs.